This invention relates generally to an electromagnetic coupling such as a clutch or brake and, more particularly, to an armature assembly for such a coupling.
Booth et al U.S. Pat. No. 5,036,964 discloses an electromagnetic clutch in which an armature is connected to a driven hub and is adapted to be frictionally coupled to a driving rotor when an electromagnet is energized to engage the clutch. When the clutch is engaged, the rotor acts through the armature to rotate the driven hub.
The armature assembly disclosed in the Booth et al patent includes a resiliently yieldable web which acts to connect a low magnetic reluctance armature disc for rotation with the driven hub. The web flexes axially in the manner of a living hinge in order to permit the armature disc to move toward and away from the rotor when the clutch is engaged and disengaged. In addition, the web yields torsionally to dampen torsional shock which occurs when the clutch is first engaged and to smooth torsional spikes which occur during steady state operation.
In the armature assembly of the Booth et al patent, the web is made of plastic and is formed with axially projecting pins which are heat staked in order to secure the armature disc to the web. In lieu of using such pins, the armature disc may be secured to the web by angularly spaced rivets each having an elongated shank and an enlarged head. The rivets form a better torque transmitting connection between the web and the disc.
When the armature disc is secured to the web by rivets, the disc is clamped against the web at the angularly spaced locations of the rivets. Due to inherent distortion of the web during molding, however, those portions of the disc located between the rivets tend to be spaced axially from the web by a narrow gap having a width typically ranging between 0.010" and 0.030". Such a narrow gap allows the web to strike the armature disc under high frequency axial vibration and, as a result, a buzz-like noise is created between the armature disc and the web.
In armature assemblies of this type, it is important that the components be captivated and prevented from flying dangerously away from the clutch in the event the driven hub locks up and causes the plastic web to be subjected to severe heat during slippage of the clutch. Under such slip and burn conditions, it is safer to allow the armature disc to break free from the web and to be captivated between the web and the rotor rather than to permit the entire armature assembly to melt away from and fly off of the hub.